The present invention relates generally to apparatus for folding sheets, and more particularly, to apparatus for forming in a sheet a fold which extends transversely to the direction of sheet travel through the apparatus. The invention is particularly useful in folding paper sheets produced in rotary roller printing presses.
Virtually all sheet folding devices of prior art configuration are designed so that a paper sheet to be folded originating from a transverse cutting of a paper is guided and transported by needles or grippers until it is engaged by a combination of a folding-blade jaw device or folding-blade folding rollers device which operates to effect guidance of the sheet. Under the influence of centrifugal forces, the cut paper sheet is guided at its front or leading edge and exhibits a tendency to be raised or lifted off a cylindrical surface upon which the sheet is supported during the folding operation. Such an approach to the folding of paper sheets involves several disadvantages. First of all, the location where a folding blade strikes the paper sheet during the folding process is generally dependent upon the speed of operation of the device and for this reason the fold to be formed shifts during operation because the paper sheet will be moved forwardly if it assumes a radius greater than the radius of the cylinder upon which it is supported. Secondly, the paper sheet will assume a speed which is greater than the circumferential speed of the folding cylinders of the device because it will move along a larger radius. This excess speed imparted to the sheet must be compensated for and as a result there will occur the formation of dog ears or cross-wrinkles in the sheet.
Accordingly, distortions in the folded sheet will occur unless the sheet is properly controlled and supported throughout the folding process.
Attemps have been made to control and appropriately guide the sheet in the process of the formation of the fold. Fixed tongues have been mounted only at a particular distance from the surface of the rotating cylinder upon which the sheet is supported in order to avoid jamming of the apparatus. However, it has been found that the sheet will still become elevated or lifted from the cylinder surface to a certain undesirable degree. Tapes or brushes rotating in synchronism and located forwardly of the juncture of the jaw cylinder or the folding rollers may operate to maintain control of the sheet but only to a point shortly before the juncture. Clamping devices on the surfaces of the cylinders of the apparatus are effective only until the end of the sheet has barely passed thereby whereupon the sheet is released. Compensation of the excess speed and catching or binding of the sheet end which attempts to move tangentially away from the cylinders is ordinarily performed by guide tongues, tapes, brushes and the like located on the discharge side of the cylinder mechanism forming the third side of a triangular chamber located between the folding cylinders of the device. It is the shape of the third side of this triangular chamber which determines whether the sheet end will be transported through the folding apparatus without cross wrinkles, dog ears or other distortions. Usually, the most favorable configuration for such a device is determined experimentally depending upon the weight and speed of the paper which is to be handled. However, when these parameters are varied, damage to the sheet must be anticipated unless the shape of the third side of the triangular chamber formed between the folding rollers or cylinders is changed in accordance with the changed parameters.
With each cross fold which is formed, the sheet edge which originally was the leading edge tends to become the trailing edge of the sheet because of a reversal of sheet direction. That is, when the folding blade jaw flap assembly or the folding blade folding roller assembly has gripped the sheet, this phenomenon tends to occur. The leading portion of the sheet is decelerated to zero and is subsequently accelerated in a reversed direction in accordance with the peripheral speed of the rotating cylinder. Such a sheet end is then generally uncontrolled to the same extent as the trailing end of the sheet and when the two ends of the sheet meet somewhere within the triangular chamber formed between the rotating cylinders, a completely uncontrolled motion of the sheet-ends tends to occur thereby causing cross wrinkles, dog ears or other distortions to arise in the configuration of the folded sheet.
The present invention is directed toward providing a folding device for forming a transverse fold in a paper sheet which is produced in a rotary roller printing press, the device of the present invention being intended to preclude the formation of undesired wrinkles, dog ears or other distortions in the sheet to be folded.